1. Field of the Invention
The invention relates to a frequency-shift keying (FSK) modulator, and more particularly, to a high-speed power-efficient coded M-ary Frequency-Shift Keying modulator adapted for circuit integration.
2. Description of the Related Art
Due to the rapid development of radio communication technology and the continuous improvement of semiconductor industries, radio communication technology is being used more extensively everywhere in daily life. Apart from popular cordless phones, cellular phones, pagers, and car alarms, a diversity of wireless products are spreading to people""s everyday life at an unprecedented speed. The constant expansion tendency makes wireless products necessarily light in weight and small in size. However, wireless products must be made at considerably low-cost while accomplishing the following features: (1) low power consumption, (2) highly efficient transmission, (3) high reception sensitivity, and (4) high anti-interference ability. If a wireless product meets the abovementioned requirements, not only radio transmission of superior quality can be achieved, but also the lifetime of an electric cell can be prolonged as to comply with environmental protection. In view of the above, we adopt conventional M-ary FSK modulation technology to design a new type of M-ary FSK modulator, which utilizes the congenital anti-interference ability associated with FSK to attain low fabrication cost, high-speed switching frequency and high efficiency of DC-to-RF output power.
Conventional FSK technologies can be categorized into the following kinds: phase-locked loop (PLL) electronic technology, microwave electromagnetic induction technology, digital direct synthesizer (DDS), and electronically controlled resonator technology.
The fundamental principle of PLL electronic technology is described as follows. The output signal of a voltage controlled oscillator in the PLL is passed through some frequency dividing circuits, and compared with a considerably stable signal by a phase comparator. Next, the phase comparator outputs a signal that is then processed by a low-pass filter to generate a slow-changing signal, which is near DC, for controlling the frequency of the output signal of the voltage controlled oscillator. By means of the feedback operation, the voltage controlled oscillator can adjust the frequency of the output signal by setting the frequency dividing circuits. The digital type setting can be achieved by utilizing FSK frequency shifting signals generated based on input electronic codes. However, a major disadvantage of the FSK frequency shifting signals obtained by PLL is that the frequency shift rate thereof is restricted by the time constant of the filter in PLL. For example, if the channel spacing of the FSK signal is 200 KHz, in order to modulate FSK signal accurately and precisely, a typical PLL time constant approximately 0.5 ms must be used, which significantly limits the FSK frequency shift rate. In addition, a typical loop bandwidth of 16.5 KHz is typically adopted, which is much greater than the data rate of FSK. Therefore, a PLL can modulate a FSK signal accurately and precisely. However, since the data rate is comparatively slow and the channel spacing of 200 KHz cannot be efficiently used, the frequency utilization rate is quite low.
Concerning the microwave electromagnetic induction technology, it utilizes a PIN diode or a varactor to transfer impedance variation to an oscillator via electromagnetic induction by way of ON/OFF motion or capacitance changes. The oscillating conditions of the oscillator are adjusted so that the frequency changes with the signal that controls the PIN diode or the varactor. The method has an advantage that the ratio of frequency shift (xcex94f) to f0 (that is, xcex94f/f0) is very small, and the shift rate is relatively high. Nevertheless, its disadvantage is that the electromagnetic circuit cannot be easily integrated, for that the size thereof approximates the wavelength of operating frequency. Therefore, the electromagnetic circuit has a comparatively large area and need to be implemented by a hybrid microcircuit. In addition to the voltage controlled varactor, a method of controlling resonator frequency by the ON/OFF motion of a PIN diode is also commonly utilized. When the PIN diode is ON (OFF), partial inductance or capacitance in the resonator can (cannot) be connected, so as to change the output frequency of the oscillator. However, the utilization of a switching circuit in a PIN diode consumes additional DC current to attain an ON state. Besides, the fabrication process of a PIN diode is unsuitable to be introduced into the common fabrication process for ICs such as CMOS transistor, bipolar transistor, or GaAs FET at the present day.
Furthermore, a digital direct synthesizer, which utilizes digital IC, is provided with a digital accumulator operating under high-speed clocked condition, saves the output signal waveform to be generated (for example, a sine wave or phase data of any composite signals) in a ROM (Read Only Memory). Take a sine wave output as an example, a digital accumulator outputs an appropriate digital code to the input end of a digital-to-analog converter (DAC) according to a designated frequency, to generate a desired analog waveform, which is then filtered through an anti-aliasing filter. The M-ary FSK signal synthesized by a digital IC requires VLSI technology; therefore the power consumed is rather large. Since the power consumption is under restriction, such technology is disadvantageous.
Lastly, an electronically controlled resonator is an oscillator which can be illustrated by FIG. 5. The partial circuit providing signal amplification in the oscillator is represented by a negative resistance (xe2x88x92R). The partial circuit responsible for resonance, represented by parallel Rr-Cr-Lr circuit, represents a parallel resonator. The Rr stands for resonator loss, and larger Rr represents lower loss. Therefore, oscillation is activated only when Rr  greater than |xe2x88x92R|. Altering the Lr and Cr values can change the oscillator frequency. The resonator in FIG. 5 is not restricted to be parallel type, series type resonator may also be used. For a series type resonator, the condition for activating oscillation is Rr  less than |xe2x88x92R|. A common way for directly modulating frequency employs a varactor to change the capacitance Cr and then modulate the frequency. Nevertheless, when a varator is used to modulate a FSK signal, the frequency shift xcex94f is often dramatically limited by the magnitude variation of the variable capacitance of the varator. This is because f0 is inversely proportional to {square root over (LrCr)}, where Cr is the equivalent capacitance of the resonator. Since the resonance frequency is inversely proportional to the square root of the equivalent capacitance Cr, a comparatively large capacitance variation is required to attain a significant frequency shift and thus enable the frequency shift function of an M-ary FSK. The other disadvantage is that, for a varactor with large capacitance variation, various bias voltage (generally much higher than the bias voltage of the oscillator) is needed, which is high in cost and is not easily integrated. Besides, a varator has a comparatively low value of Q among integrated circuits, which results in large loss. In the U.S. Pat. No. 6,078,226 entitled xe2x80x9cIntegrated Circuit Implementation of a Frequency Shift Keying Oscillatorxe2x80x9d, an integrated circuit of an M-ary FSK is disclosed, as shown in FIG. 6. The circuit utilizes a power supply to switch an FSK oscillator, and employs switches to change the reactance of a SAW resonator in order to achieve high-speed switching operation. However, the circuit utilizes a single SAW resonator wherein multiple switches are used to change the reactance to attain the operation of an M-ary FSK. Since the range of frequency shift is significantly limited by the SAW resonator in the method, the frequency shift (xcex94f) cannot be very large, as it restricts data transmission speed.
In view of the aforesaid problems, an object of the invention is to provide a high-speed power-efficient coded M-ary FSK modulator. The high-speed power-efficient coded M-ary FSK modulator in accordance with the invention includes: a coding logic, which generates (M/2) Gray code signals in accordance with an (Nxe2x88x921) bits control signal; and (M/2) switching oscillators. Each switching oscillator includes: a composite crystal resonator with a first end and a second end; a first switch, which is controlled by Gray code signals, with one end connected to the first end of the composite crystal resonator, and the other end grounded via an equivalent negative resistance circuit; a second switch, which is controlled by serial data, with one end connected to the second end of the composite crystal resonator, and the other end grounded; and a capacitor, with one end connected to the second end of the composite crystal resonator, and the other end grounded.
Whereby one of the (M/2) switching oscillators is activated by a control signal, and the frequency of the activated oscillator is switched by the serial data for achieving M-ary frequency shift.